This invention relates to a thermostatic control, and more particularly to a bimetal thermostatic switch particularly adapted to control the operation of the compressor motor of a refrigerator or the like thereby by closely maintain the temperature within a refrigerator compartment within a predetermined range (e.g., 40.degree.F .+-. 2.degree.F).
In controlling the temperature within a refrigerator, it is important to insure that the temperature is accurately maintained within a relatively narrow range, such as specified above. If the temperature is allowed to drift appreciably above this range, spoilage of food may be hastened. If the temperature drifts below the 32.degree.F, certain foods will, of course, freeze. In the past, so-called "change-of-state" temperature controls were widely used in refrigerators. These controls utilized a fluid expansion bulb (e.g., a capillary tube) having a fluid sealed therein which underwent a change in state (e.g., from a vapor to a liquid or vice versa) at a predetermined temperature (which depended on the pressure of the gas) and thus actuated a bellows-type actuator which in turn mechanically tripped an electric switch. These change-of-state controls are reliable and capable of being accurately calibrated to repeatedly switch the compressor motor on and off so as to maintain the temperature of the refrigerator within the predetermined range. However, these change-of-state switches are expensive and are relatively complicated.
Heretofore, bimetal thermostatic controls have widely been used as room air conditioner controls. Reference may be made to such U.S. Pat. Nos. as 2,074,132, 3,293,875, Re. 26,554, and the co-assigned U.S. Pat. No. 3,546,652 for examples of such room air conditioner controls. Generally, these controls include a bimetal thermostatic strip adjustably mounted so as to open or close the electrical contacts of a switch to make and break a circuit supplying electrical power to the air conditioner's compressor motor. In the above mentioned U.S. Pat. Nos. 3,293,875 and 3,546,652 provision is made for adjusting the bimetal strip to open and close the switch contacts within the switch at any desired temperature within a limited operating range.
However, it has been found that these room air condition bimetal controls are not wholly suitable as cold controls for refrigerators. Refrigerators has been developed to the point where they have a reliable operating life of 10 to 15 years, and temperature controls must be capable of reliably switching the compressor motor on and off many times a day under severe humidity and other environmental conditions within the refrigerator for the life of the refrigerator (e.g., 750,000 cycles may be required). Since bimetal thermostatic switches utilize the thermal flexure of a bimetal element to sense temperature changes, the narrow temperature ranges in which the temperature of the refrigerator must be maintained require that the switch must be sensitive to small movements of the bimetal element. The bimetal cold controls, of necessity, must be made of insulative material so as to electrically insulate parts of the switch. Over long periods of time and exposure to high humidity, these insulative materials may undergo slight dimensional changes which significantly alter the amount of movement of the bimetal required to actuate the switch. Thus, over a long period of time (e.g., several years), the temperatures at which the compressor motor is energized and de-energized may change or drift appreciably. The temperature drift of a control should be less than plus or minus 2.degree.F. over the life of the refrigerator. Prior bimetal controls experienced significantly more drift than this.
Also prior bimetal controls were sometimes exposed to relatively high or low ambient temperatures during shipping which, under some circumstances, caused the bimetal element to be overstressed and thus to yield causing it to lose its calibration. More specifically, bimetal switches are conventionally shipped in their locked off position in which the bimetal element is mechanically stressed to exert a maximum force on the switch thereby to maintain the switch open during shipping. While in transit, this switch may experience ambient temperatures between + 150.degree.F. and - 50.degree.F. These temperature extremes may superimpose a thermal stress on the bimetal element which is additive to the mechanical stress previously applied thereto and thus may cause the bimetal element to permanently yield.